In most refrigeration and air conditioning systems, oil is circulated with the refrigerant as part of the lubrication and cooling process. Circulation is greatest at startup. Because oil interferes with heat transfer in the system, the oil is typically removed from the refrigerant prior to its reaching the heat transfer elements of the system although some oil is still present after separation. The oil separation may take place within the shell of a high side hermetic compressor or may take place downstream of the compressor. Oil viscosity increases with decreasing temperature and refrigerant tends to migrate to the coldest part of the system so that in situations where the compressor/oil separator is exposed to cold temperatures it is often desirable to heat the oil to prevent refrigerant migration to the compressor sump/oil separator, for easier compressor starts and/or to reduce the size of the motor required. Thus, oil is diluted if migration of refrigerant can take place and is entrained in the refrigerant and transported in the system and may thus be removed from the area of need. It has become a standard procedure to monitor the level of the oil in a sump and to shut off the compressor when the oil level drops to a predetermined level. Additionally, the provision of an oil sump heater is a standard option where refrigerant migration is a problem and/or where oil viscosity may become too great.